Such apparatus is often used in place of a vibration screen which has various disadvantages, such as the development of noise and dirt. German Offenlegungsschrift No. 2,518,112 describes an apparatus including a housing having either a horizontal or a tilted axis and which contains a rotation-symmetrical screen basket. Inside the basket, there is a rotating shaft that is provided with conveyor elements that are arranged coaxially to the basket axis. A supply line for the fiber suspension which is to be cleaned leads into the interior of the screen basket at one end of the basket. An outlet opening for deposited impurities is arranged at the other end of the basket. A discharge pipe for the cleaned suspension is arranged between the screen basket and the housing wall. In the apparatus in the German publication, the impurities are deposited by a type of worm or screw thickener having a tilted axis. The suspension which is to be cleaned is fed into the interior of the screen basket at the lower end. Liquid and fibers pass through the screen basket and the impurities which are to be deposited are transported upward by the worm shaft and are removed via the impurities outlet opening.
A disadvantage of the above-described device is that either transport difficulties occur in respect of the impurities which are to be deposited, or insufficient water extraction occurs. If the liquid level in the housing is set to be low, although good dewatering takes place, the transportation of the impurities to the impurities outlet opening presents difficulties since the numerous small particles are difficult to transport with the conveyor worm. If, on the other hand, the liquid level is set to be sufficiently high to improve the transportation of impurities, the impurities are inadequately thickened before they are removed from the housing via the impurities outlet opening.